The present invention relates to water-soluble or water-swellable crosslinkable copolymers based on ammonium salts of acrylamidoalkylsulfonic acids and cyclic N-vinylcarboxamides or cyclic and linear N-vinylcarboxamides, to the preparation thereof and to the use thereof as thickeners, stabilizers of emulsions and dispersions and as glidants in cosmetic and pharmaceutical compositions.
Water- or solvent-containing multicomponent systems, such as solutions, emulsions or suspensions, are frequently adjusted to higher viscosities or thickened for economical or performance reasons, or for stability reasons. Thus, for example, by increasing the viscosity of the external or internal phase of emulsions or suspensions, it is possible to significantly prolong the time before the components of such a system separate, which is evident from an extension of the shelf life. For many products, increasing the viscosity also improves their ability to be spread uniformly, in particular on uneven surfaces. This is true in particular for skincare compositions and pharmaceutical ointments on the skin. In the case of many industrial products, such as wallpaper stripping agents, paint strippers or aircraft de-icers, the increased viscosity prevents premature run-off from the surface to be treated. The more uniform distribution and extended contact time thus increase the effectiveness. As well as the performance advantages mentioned, the high viscosity of such preparations also offers further advantages during the preparation, packaging, containerizing and storage, as well as during transportation, the thickening of acidic media being of particular importance here from a safety viewpoint. In general, the rheological properties during the preparation and/or formulation of cosmetic, pharmaceutical or industrial preparations are a decisive criterion for the use of these products in practice. Even when used in extremely small amounts, the thickeners employed should lead to adequate thickening. However, the color and principal properties of the medium to be thickened should not be changed.
To adjust the rheological properties of aqueous or solvent-containing systems, emulsions, suspensions, a large number of different systems are given in the specialist literature. Known examples are cellulose ethers and other cellulose derivatives (e.g. carboxymethylcellulose, hydroxyethylcellulose), gelatin, starch and starch derivatives, sodium alginates, fatty acid polyethylene glycol esters, agar agar, tragacanth or dextrins. The synthetic polymers used are various materials, such as e.g. polyvinyl alcohols, polyacrylamides, polyacrylic acid and various salts of polyacrylic acid, polyvinylpyrrolidone, polyvinyl methyl ether, polyethylene oxides, copolymers of maleic anhydride and vinyl methyl ether, and various mixtures and copolymers of the compounds given above.
However, said compounds exhibit diverse disadvantages upon use. Thus, for example, the cellulose derivatives and, generally, the materials based on natural raw materials and the formulations resulting therefrom are very susceptible to bacteria. From an applications-related viewpoint, they are mostly noticeable from the formation of unpleasant xe2x80x9cstringingxe2x80x9d gels. Fatty acid polyethylene glycol esters tend toward hydrolysis in the presence of water, and the insoluble fatty acids which form in the process cause undesired clouding. Thickeners of natural origin (e.g. agar agar or tragacanth) have a composition which varies considerably depending on their origin.
EP-A-0 816 403 and WO 98/00094 describe crosslinked homopolymers of 2-acrylamido-2-methylpropanesulfonates and the use thereof as thickeners. EP-A-0 510 246 describes crosslinked copolymers of N-vinylcarboxamides and unsaturated alkylamides substituted by a sulfonate group, which are likewise suitable as thickeners. U.S. Pat. No. 5,080,809 describes non-crosslinked copolymers of N-vinylpyrrolidone and 2-acrylamido-2-methylpropanesulfonate. DE 199 05 639.0 describes crosslinked polymers of noncyclic N-vinylcarboxamides and acrylamidoalkylsulfonic acids.
Surprisingly, we have now found that alkali metal, alkaline earth metal or ammonium salts of various acrylamidoalkylsulfonic acids are sufficiently soluble in solvents acceptable for cosmetic applications, such as alcohols or alcohol mixtures, and are therefore highly suitable for a copolymerization with cyclic N-vinylcarboxamides which are likewise soluble in these solvents, or mixtures of two or more cyclic N-vinylcarboxamides or mixtures of cyclic and linear N-vinylcarboxamides, optionally with further monomers, and monomers which act as crosslinkers. In contrast to this, according to the prior art, it is obligatory to work in an aprotic solvent. Since the ammonium salt of 2-acrylamido-2-methylpropanesulfonic acid, which is preferably used for the polymerization, is in ionic form the crosslinked copolymer obtained no longer needs to be subsequently neutralized in an involved manner, but can be used as a thickener immediately following polymerization and removal of the solvent. A further advantage is that through appropriate choice of the comonomer(s) (cyclic N-vinylcarboxamides, mixtures of cyclic and linear N-vinylcarboxamides) it is possible to control the ratio of ionic to neutral building blocks and thus to regulate the thickening action and salt stability and better match them to specific requirements. Furthermore, as result of the polymerization in alcohol or alcohol mixtures with a water content of less than 10% by weight and here in particular in tert-butanol, products are obtained which, with regard to their residual content of solvent remaining in the product, are toxicologically safe and can thus be used, for example, in cosmetic products.
The invention provides crosslinked copolymers consisting essentially of
a1) 1 to 50% by weight of the repeating structural unit of the formula (1) 
where n is an integer from 2 to 9,
or
a2) 1 to 50% by weight of a mixture of the repeating structural unit of the formula (1) and of the repeating structural unit of the formula (2) 
where R, R1 and R2 may be identical or different and are hydrogen or a linear or branched alkyl or alkenyl group having in each case 1 to 30, preferably 1 to 20, in particular 1 to 12, carbon atoms and
b) 49.99 to 98.99% by weight of the repeating structural unit of the formula (3) 
in which R3 is hydrogen, methyl or ethyl, Z is C1-C8-alkylene, n is an integer from 2 to 9, and X is an alkali metal or alkaline earth metal ion, and
c) 0.01 to 8% by weight, preferably 0.01 to 5% by weight, of crosslinking structures resulting from monomers having at least two olefinic double bonds.
Preferred copolymers according to the invention contain 2 to 30% by weight, in particular 3 to 15% by weight, of structural units of the formula (1), or (1) and (2), preferably derived from N-vinylpyrrolidone, 69.5 to 97.5% by weight, in particular 84.5 to 96.5% by weight, of structural units of the formula (3), preferably derived from the ammonium salt of 2-acrylamido-2-methylpropanesulfonic acid and 0.2 to 3% by weight, in particular 0.5 to 2% by weight, of crosslinking structures resulting from monomers having at least two olefinic double bonds. The mixing ratio of the monomers forming the basis of structural units 1 and 2 can be varied within any desired limits.
Crosslinking structures resulting from monomers having at least two olefinic double bonds are preferably derived from allyl acrylate or allyl methacrylate, dipropylene glycol diallyl ether, polyglycol diallyl ether, triethylene glycol divinyl ether, hydroquinone diallyl ether, tetraallyloxyethane or other allyl or vinyl ethers of multifunctional alcohols, tetraethylene glycol diacrylate, triallylamine, trimethylolpropane diallyl ether, methylene bisacrylamide or divinylbenzene. The crosslinking structures are particularly preferably derived from monomers of the formula (4), 
in which R is hydrogen, methyl or ethyl.
The copolymers according to the invention are prepared by dissolving or dispersing the monomers corresponding to the repeating structural units of the formulae (1), (2) and (3) in a protic solvent, adding one or more crosslinkers having at least two olefinic double bonds to this solution or dispersion, and starting the polymerization in a manner known per se by adding a free-radical-forming compound.
Preference is given to copolymerizing the ammonium salt of acrylamidopropanesulfonic acid. Instead of this ammonium salt, it is also possible to use the free acrylamidopropanesulfonic acid and, before adding the remaining monomers, producing the ammonium salt from the free acid by introducing ammonia.
The polymerization reaction is preferably carried out in a water-soluble alcohol or a mixture of two or more alcohols having 1 to 6 carbon atoms, preferably in tert-butanol. The water content of the alcohol or of the mixture of two or more alcohols must not exceed 10% by weight since otherwise the formation of lumps can occur over the course of the polymerization. Specifically, the type and amount of solvent are chosen such that the salt of acrylamidoalkylsulfonic acid of the formula 1, in particular of 2-acrylamido-2-methylpropanesulfonic acid, is largely soluble or dispersible therein. Largely soluble or dispersible is understood as meaning that even after the stirrer has been switched off, no solid material settles out of the solution or dispersion. By contrast, the polymer formed in the course of the reaction should be largely insoluble in the chosen solvent (or solvent mixture). Largely insoluble is understood here as meaning that in the course of the polymerization a readily stirrable pulpy polymer paste is produced in which no lumps or agglutinations must form. The filtrate which is obtainable by filtering the paste with suction must have a solids content of at most 5% by weight. If the copolymers are soluble in the chosen solvent or solvent mixture to a greater extent, clumping may result during drying of the polymer paste. The polymerization reaction itself is triggered in a manner known per se by free-radical-forming compounds, such as azo initiators (e.g. azobisisobutyronitrile), peroxides (e.g. dilauryl peroxide) or persulfates in a suitable temperature range from 20 to 120xc2x0 C., preferably between 40 and 80xc2x0 C., and is continued over a period of from 30 min to several hours.
The copolymer composition can be varied by varying the above-described ratio of the monomers used, and the proportion of crosslinker and thus be used to achieve a tailored profile of properties. For example, by incorporating more ammonium salts of acrylamidosulfonic acids, it is possible to improve the thickening action of the polymers, while by incorporating more cyclic N-vinylcarboxamide, it is possible to improve the electrolyte compatibility of the polymers and the solubility thereof in nonaqueous systems.
In contrast to polymers based on acrylic acid which, in the neutral or slightly alkaline range in 1% strength aqueous solution, exhibit viscosities of more than 30 000 mPaxc2x7s, but whose thickening ability (or the measured viscosity) deteriorates considerably with decreasing pH, the copolymers described according to the invention are able to maintain their viscosity up to an acidic pH of about 3.